Value documents are understood here to be sheet-shaped objects that represent for example a monetary value or an authorization and hence should not be producible arbitrarily by unauthorized persons. Hence, they have features that are not simple to produce, in particular to copy, whose presence is an indication of authenticity, i.e. manufacture by an authorized body. Important examples of such value documents are chip cards, coupons, vouchers, checks and in particular bank notes.
Value documents often have an optical design specific to the respective type of the value documents, i.e. a specific appearance, which can, but does not have to, additionally comprise security features. The optical design can be given for example by a printed image on the value document. The printed image here can be a printed image in the range of the optical radiation or the optical wavelength range, which will hereinafter be understood to be infrared radiation, ultraviolet radiation and radiation in the visible wavelength range.
For a given value document, the optical design, in particular the printed image, can deviate from the specified design, for example because the value document has been soiled through use or tampering. The nature and extent of the deviations in the optical design substantially determine the optical condition of a value document, which in turn constitutes an essential aspect of the overall condition of a value document. The condition of the value document can determine in particular whether or not a value document can still be employed as such, i.e. is fit for circulation. With respect to the optical condition, the deviations, in particular in the printed image, can for example, depending on their nature and extent, impair or even exclude the recognition of the value or authenticity of a value document. Hence, it is important to be able to examine and rate the condition of a value document by machine if possible. In view of the many possibilities for a deviation of the optical design from the specified design, it is desirable to know methods for examining the optical condition of value documents that enable a differentiated rating.